In communication systems, a signal to be transmitted is typically concentrated around a particular carrier frequency occupying a defined channel. Information is sent in the form of modulation of frequency, envelope or phase or some combination of these which causes the information to be represented by energy spread over a band of frequencies around the carrier frequency. In many schemes the carrier itself is not sent since it is not essential to the communication of the information.
Modulation schemes which vary the envelope and phase of the carrier such as the Quaternary Phase Shift Keying (QPSK) and the Quadrature Amplitude Modulation (QAM) have high spectral efficiency and are suitable for multi-user systems transmitting a large amount of information in a limited frequency bandwidth. When the modulation scheme generates a varying envelope, the transmitter must be extremely linear to preserve the narrowbandness of the output signal. The most crucial block which determines the linearity of the transmitter is the power amplifier (PA).
Many communication systems, such as mobile multi-user systems impose stringent requirements on the linearity of the PA. For this reason, linear PA architectures such as class A, class AB, class B, and class C are commonly used. However, the efficiency of such PAs at high output power levels is limited and they become non-linear causing Adjacent Channel Interference (ACI) and limiting the spectral efficiency of the communication system. Linearization techniques such as Cartesian feedback, adaptive predistortion and feedforward have been used to improve the linearity of the PAs in such applications. However, these methods add considerably to the complexity and the power dissipation of the transmitter and reduce the overall efficiency of the PAs making them unsuitable for use in hand held and portable devices, where battery size and life are of jugular importance.
A frequent goal was to design a linear transmitter architecture for spectrally efficient communication systems to overcome the low transmitter efficiency caused by using linear PAs. To achieve the objective, a system, known as the outphasing modulation, was developed in the 1930's for Amplitude Modulation (AM) broadcast transmitters and methods based on this concept were later developed including the LINC (LInear amplification using Non-linear Components) and CALLUM (Combined Analog Locked-Loop Universal Modulator) systems. The essence of these techniques lies in the realization that any envelope and phase modulated signal can be represented by the summation of two components with fixed envelopes but varying phases. The advantage of this transformation is that each phase modulated component can be amplified using highly non-linear yet power efficient techniques without generating Adjacent Channel Interference (ACI). There exist many credentials that discuss the above-mentioned considerations. The U.S. Pat. No. 6,054,894, to Wright et al. as well as D. C. Cox's “Linear Amplification With Nonlinear Components,” IEEE Transactions on Communication, vol. COM-22, pp.1942-1945, 1974, illustrate the LINC transmitter, where the U.S. Pat. No. 6,366,177, to McCune et al. and A. Bateman's “The Combined Analog Locked Loop Universal Modulator (CALLUM),” Proceeding of the 42nd IEEE Vehicular Technology Conference, pp. 759-764, 1992, illustrate the CALLUM transmitter.
The concept of the LINC transmitter is to represent an envelope varying signal by two constant envelope signals with varying phases and amplify them separately using non-linear amplifiers. These two signals are then added together to produce a linear amplified replica of the input signal.
FIG. 1 illustrates the notion of the LINC transmitter. The baseband Digital Signal Processor (DSP) 101 is used to process the input data 110 and generate two quadrature signals 111 si(t) and sq(t), where si(t) represents the in-phase component and sq(t) represents the out-of-phase or quadrature component. The IF modulator 102 generates the Intermediate Frequency (IF) signal 112. The Signal Component Separator (SCS) 103 is used to convert the IF signal 112 into two constant envelope signals 113 s1IF(t) and s2IF(t), which are phase modulated, by adding and subtracting a mediator signal to and from the IF signal 112. The mediator signal can be obtained from the input IF signal 112.
The implementation of the SCS 103 has been one of the major problems in the realization of the LINC concept because of the high frequency operation. As a result, the SCS 103 usually operates at Intermediate Frequencies (IFs) rather than radio frequencies (RFs) and the two constant envelope signals 113 must be translated to RF using upconverters 104, as illustrated in FIG. 1, prior to transmission from the antenna 106. The two upconverters 104 add to the power dissipation of the transmitter and generate Intermodulation Distortion (IMD) affecting the overall linearity of the transmitter. Furthermore, the SCS 103 may not accurately operate over the full bandwidth of the IF signal 112 and may introduce gain and phase misalignments between the two constant envelope signals 113. Such misalignments appear as non-linearities at the output of the LINC transmitter 114.
CALLUM is an alternative technique which can provide the constant envelope signals. The CALLUM system uses a feedback architecture and employs two Voltage Controlled Oscillators (VCOs) controlled by reference signals to generate the phase modulated constant envelope signals. The reference signals are obtained by comparison of the quadrature components of the input and the downconverted output at baseband.
Another alternative used to achieve the same objectives as LINC and CALLUM is the Vector Locked Loop (VLL) system. The VLL also utilizes VCOs as does the CALLUM, but requires signal processing using polar (magnitude and phase) rather than Cartesian (in-phase and out-of-phase) signals.
To generate the two components of the output at baseband frequency using either CALLUM or VLL, requires a downconverting mixer operating at RF which adds to the complexity and the power dissipation of the transmitter. In addition, the non-linearities caused by the downconverting mixer reduce the accuracy of the reference signal generation. The accurate operation of the CALLUM and VLL systems is also limited to a narrowband spectrum because of the feedback configuration which has a limited loop capture range and instability at high loop gains.
Although the prior art overcomes some of the drawbacks mentioned above, they still require upconverters, downconverters, a feedback system or an SCS, or some combination of these which complicate the design and increase the power dissipation.